Profile adjustment in plasma ion implanter

ABSTRACT

A method to provide a dopant profile adjustment solution in plasma doping systems for meeting both concentration and junction depth requirements. Bias ramping and bias ramp rate adjusting may be performed to achieve a desired dopant profile so that surface peak dopant profiles and retrograde dopant profiles are realized. The method may include an amorphization step in one embodiment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part of U.S. applicationSer. No. 11/376,522 entitled “Profile Adjustment in Plasma IonImplanter,” filed Mar. 15, 2006, which claims priority to U.S.Provisional Application No. 60/662,018 entitled “Profile Adjustment inPlasma Ion Implantation,” filed Mar. 15, 2005, each of which are herebyincorporated by reference herein in its entirety.

TECHNICAL FIELD

The described apparatus and methods relate generally to providingprofile adjustment solutions in plasma doping (PLAD) applications tomeet both concentration and junction depth requirements.

BACKGROUND OF THE INVENTION

Plasma doping systems are known and used for forming shallow junctionsin semiconductor wafers and for other applications requiring highcurrent with relatively low energy ions. In plasma doping systems, asemiconductor wafer is placed on a conductive platen, which functions asa cathode and is located in a plasma doping chamber. An ionizable dopinggas is introduced into the chamber, and a voltage pulse is appliedbetween the platen and an anode or the chamber walls, causing formationof a plasma containing ions of the dopant gas. The plasma has a plasmasheath in the vicinity of the wafer. The applied pulse causes ions inthe plasma to be accelerated across the plasma sheath and to beimplanted into the wafer. The depth of implantation is related to thevoltage applied between the wafer and the anode. Very low implantenergies can be achieved. Examples of such plasma doping systems aredescribed in U.S. Pat. No. 5,354,381 to Sheng, U.S. Pat. No. 6,020,592to Liebert et al., and U.S. Pat. No. 6,182,604 to Goeckner. In the abovedescribed plasma doping systems, the applied voltage pulse generates aplasma and accelerates positive ions from the plasma toward the wafer.In other types of plasma systems, a continuous plasma is produced, forexample, by inductively-coupled RF power from an antenna locatedinternal or external to the plasma doping chamber. The antenna isconnected to an RF power supply. At intervals, voltage pulses areapplied between the platen and the anode, causing ions in the plasma tobe accelerated toward the wafer.

Dopant gas species used for plasma implantation may decompose ordissociate during the implant process into atomic or molecular fragmentswhich may be deposited on the surface of the wafer. Atomic or molecularfragments that result from dissociation of dopant gas molecules arereferred to herein as “neutral particles.” Examples of dopant gasspecies which dissociate during the implant process include AsH₃, PH₃,BF₃, and B₂H₆. For example, arsine gas AsH₃ may dissociate into As, AsHand AsH₂, which may be deposited on the surface of the wafer beingimplanted. These deposited surface layers can cause a number ofproblems, including dose non-repeatability, poor dose uniformity anddose measurement problems. In particular, the neutral particles thatform the deposited surface layers are not measured by the dosemeasurement system. Further, the depth profile of the dopant is alteredby the deposited surface layer itself and by its effect on implantedions. In addition, the deposited surface layers can cause contaminationof other equipment, such as annealers, when the wafers are subsequentlyprocessed in such equipment.

Accordingly, there is a need for providing a profile adjustment solutionin plasma doping applications to meet both concentration and junctiondepth requirements.

SUMMARY OF THE INVENTION

The invention includes methods and apparatuses for providing a dopantprofile adjustment solution in plasma doping systems for meeting bothconcentration and junction depth requirements. Bias ramping and biasramp rate adjusting may be performed to achieve a desired dopant profileso that shallow and abrupt junction in vertical and lateral directionsare realized that are critical to device scaling in plasma dopingsystems. The ramping of the implanting voltage bias may be linear ornon-linear ramping. The rate of ramping the implanting voltage bias maybe adjusted and the rate may vary with respect to the deposition rate.Specifically, the rate of ramping the implanting voltage bias may befaster, slower or match the deposition rate. The implanting voltage biasmay be ramped in combination with ramping the duty cycle and incombination with changing at least one implant process parameter. Theramping and adjustments are performed to maximize the retained dose andnear surface concentration while minimizing dopant spread in verticaland lateral directions.

A first aspect of the invention is directed to a method for plasmaimplantation of a workpiece comprising the steps of introducing a dopantgas into a plasma doping chamber and ramping an implanting voltage biasto accelerate the dopant gas ions toward the workpiece. The implantingvoltage bias is ramped to maximize the retained dose and near surfaceconcentration of the implanted dopant gas ions into the workpiece.

A second aspect of the invention is directed to a plasma dopingapparatus comprising a plasma doping chamber, a platen, a gas source anda voltage source for implanting dopant gas ions into a workpiece. Thevoltage source generates an implanting voltage bias to accelerate thedopant gas ions from the plasma toward the workpiece that is ramped tomaximize the retained dose and near surface concentration of theimplanted dopant gas ions into the workpiece.

The foregoing and other features of the invention will be apparent fromthe following more particular description of embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of this invention will be described in detail, withreference to the following figures, wherein like designations denotelike elements, and wherein:

FIG. 1 is a block diagram of a plasma doping system according to anembodiment of the present invention;

FIGS. 2( a) and 2(b) are graphs of implant ramping bias voltagesaccording to embodiments of the present invention;

FIG. 3 shows SIMS and Rs data comparison for a five step bias rampingaccording to one embodiment of the present invention;

FIG. 4 shows a dopant profile for bias ramping according to oneembodiment of the present invention;

FIGS. 5( a), 5(b), and 5(c) show bias ramping combined with duty cycleramping according to embodiments of the present invention;

FIG. 6 shows effects on the dopant profile for various deposition ratestied with the bias ramping according to embodiments of the presentinvention;

FIG. 7 shows a junction profile for reverse bias ramping according to anembodiment of the present invention;

FIG. 8 shows a junction profile for bias ramping with a process havingan initial deposition according to an embodiment of the presentinvention;

FIG. 9 shows a retrograde junction profile;

FIG. 10 shows a ramping bias voltage and resulting retrograde junctionprofile; and

FIG. 11 shows a non-ramping bias voltage and resulting surface peakdopant profile.

DETAILED DESCRIPTION OF THE INVENTION

A box-shape dopant profile is desirable in ion implantation processesfor semiconductor device manufacturing. For mono-energetic incidentions, such as those from a beamline implanter, the dopant profile istypically Gaussian. PLAD profiles tend to peak near the surface, withthe tail of its profile approaching the beamline tail of the sameimplant energy. In applications where a certain profile shape isdesired, profile adjustment can be made by changing implant energy anddose. In PLAD applications, surface deposition may occur during theimplant and by applying a bias voltage that changes with depositionthickness a box-like profile may be achieved. According to oneembodiment of the present invention, a method is described to controlthe ramping and/or rate of ramping the bias voltage in PLAD applicationsto maximize retained dose and near surface concentration, whileminimizing dopant spread in vertical and lateral directions.

During plasma implant processes, deposition typically occurs on thewafer surface due to neutrals and low energy ions from the plasma. Thisdeposition progressively increases through the implant from zero at thebeginning of the implant to a small value, 100 Angstroms for instance,towards the end of a high dose implant, such as 2E16 for instance. Thedepositing film on the wafer surface may impede the penetration depth ofthe implanting ions which may make the ion implant progressivelyshallower through the implant process. One embodiment of the presentinvention compensates for increasing deposition by progressivelyincreasing the implant voltage through the implant step. Suchcompensating accounts for the fact that the deposition rates aredifferent in different chemistries and conditions. The compensating maybe achieved by either a pre-programmed software look up table or anin-situ monitor device installed in the implanting system. By varyingthe thickness of surface deposition through the plasma implant step, theimplant depth of the ions may change accordingly through the plasmaimplant step. The implant characteristics are thereby changed to makethe implant sensitive to dose, bias frequency, pulse width and otherprocess parameters that impacts the implant time.

For typical implant processes, an increase of dopant concentration nearthe surface may cause a proportional increase at the tail regionprofile. Junction depths, both vertical and lateral, are sensitive toion energy and tail concentration. A deep junction usually negativeimpacts on device performance. For applications that require a highdopant concentration near the surface, a high implant dose is needed butthe implant energy is limited by the junction target. One problem forlow energy, high dose implants is the resulting low wafer throughput. InPLAD, low energy ions can also be blocked by surface deposition, leadingto low retained dose problems. High bias voltages may satisfy the dosetarget but not the junction target.

High dose or surface concentration is needed to achieve low contactresistance and low spreading resistance. Shallow and abrupt junctions invertical and lateral directions are critical to device scaling.According to one embodiment of the present invention, a profileadjustment solution for PLAD is provided to meet both concentration andjunction depth needs. One example of a plasma ion implantation systemsuitable for the embodiments of the present invention is illustrated inFIG. 1. A process chamber 10 defines an enclosed volume 12. A platenpositioned within the process chamber 10 provides a surface for holdinga substrate, such as a semiconductor wafer 20. An anode 24 may bepositioned with the process chamber 10 in spaced relation to the platen14. The anode 24 may be connected to electrically conductive walls ofthe process chamber 10, both of which may be connected to ground.Alternatively, the platen may be connected to ground, and the anode 24pulsed to a negative voltage. In further embodiments, both the anode 24and the platen 14 may be biased with respect to ground. Thesemiconductor wafer 20 and the anode may be connected to a high voltagesource 30 via the platen 14 so that the semiconductor wafer 20 functionsas a cathode. The voltage source 30 may provide pulses in a range ofabout 20 to 20,000 volts in amplitude, for about 1 to 200 μseconds and apulse repetition rate of about 100 Hz to 20 kHz. However, it should beunderstood that these pulse parameter values are given by way of exampleonly and other values may be utilized within the scope of the presentinvention by those skilled in the art.

A controller 50 regulates the rate at which gas is supplied from a gassource (not shown) to the process chamber 10 for supplying an ionizablegas containing a desired dopant for implantation into the semiconductorwafer 20. This configuration provides a continuous flow of process gasat a desired flow rate and constant pressure. It should be realized bythose skilled in the art that other configurations may be utilized forregulating gas pressure and flow. A thickness detector 52 communicateswith the process chamber 10 and provides the detected information to thecontroller 50. The thickness detector 52 may be an in-situ filmthickness monitor based on reflectivity, however, other known sensorsmay be used for observing the wafer surface for the deposition rate. Theplasma ion implantation system may also include additional components,depending on the configuration of the system. The system typicallyincludes a process control system (not shown) which controls andmonitors the components of the plasma ion implantation system toimplement a desired implant process. Systems which utilize continuous orpulsed RF energy include an RF source coupled to an antenna or aninduction coil. The system may also include magnetic elements whichprovide magnetic fields that confine electrons and control plasmadensity and spatial distribution.

In operation, the semiconductor wafer 20 is positioned on the platen 14and the pressure control system produces the desired pressure and gasflow rate within the process chamber 10. The voltage source 30 applies aseries of high voltage pulses to the semiconductor wafer 20, causingformation of a plasma 40 in a plasma discharge region 48 between thesemiconductor wafer 20 and the anode 24. The plasma 40 contains positiveions of the ionizable gas and includes a plasma sheath 42 in thevicinity, typically at the surface, of the semiconductor wafer 20. Theelectric field that is present between the anode 24 and the platen 14during the high voltage pulse accelerates positive ions from the plasma40 across the plasma sheath 42 toward the platen 14. The acceleratedions are implanted into the semiconductor wafer 20 to form regions ofimpurity material. The pulse voltage is selected to implant the positiveions to a desired depth in the semiconductor wafer 20. The number ofpulses and the pulse duration are selected to provide a desired dose ofimpurity material in the semiconductor wafer 20. The current per pulseis a function of pulse voltage, pulse width, pulse frequency, gaspressure and species and any variable position of the electrodes. Forexample, the cathode to anode spacing may be adjusted for differentvoltages.

As noted above, dopant gas species typically used for plasmaimplantation may dissociate into neutral particles during the implantprocess and form deposited surface layers on the semiconductor wafer 20.Examples of dopant gas species which form deposited surface layersinclude AsH₃, PH₃ (phosphine) and B₂H₆. Some fluorides such as BF₃ mayform deposit surface layers under certain plasma doping conditions. Forexample, arsine gas may dissociate into As, AsH and AsH₂, which may bedeposited on the surface of the semiconductor wafer 20. Similarly, BF₃may dissociate into B, BF and BF₂ which may be deposited on the surfaceof the semiconductor wafer 20. These deposited surface layers cause dosenon-repeatability, poor dose uniformity and metrology problems.

In the present embodiment, the deposition rates as a function ofchemistry and plasma doping conditions are characterized. The stoppingpower of the deposition layer to the incident ions are alsocharacterized. A first implant bias (V1) is used to complete apredetermined first target dose, allowing a first deposition layer to begrown during the first implant period t1 as shown in FIG. 2( a). Next asecond implant bias (V2) is used to complete a second dose, with thesecond implant energy adjusted according to the deposition layerthickness. Subsequent implant bias and times are repeated until thetotal implant dose is reached. The number of iterations, n, may beselected according to the particular application. In characterizing thedeposition rates as a function of chemistry and plasma dopingconditions, a knowledge base may be developed. The plasma doping systemmay access this knowledge base for estimating the deposition rate whichdepends on the recipe required by the implantation process. The implantbias voltage or bias frequency are typically ramped up through theimplant process to compensate for the deposition which starts at zeroand progressively increases as the implant progresses.

In another embodiment of the present invention, the bias voltage isadjusted continuously, following either a linear or non-linear curve,until the total dose is reached as illustrated in FIG. 2( b). A feedbackcontrol system, including the in-situ film thickness detector 52 andother process monitors may be used so that the increase of implant biasor frequency is just enough for ions gaining extra momentum to penetratethe surface film. The amount of bias increase may be more or less thanwhat is needed to compensate for the deposition layer to achieve adesired dopant profile.

In one implementation of the present invention, the method is tested ina B₂H₆ plasma doping system with a five-step bias ramping, from 4 kV to6 kV, with 0.5 kV steps. In FIG. 3, SIMS results show retained dose andsurface concentration similar to 6 kV, but junction depths shallowerthan at 5 kV and these results can be confirmed by Rs data. Theembodiments of the present invention may also be implemented for PLADprocesses that require a box-like profile, such as ultra-shallowjunction (USJ) formation, source drain extensions (SDE), source drainand poly gate doping, and material modification using high dose, lowenergy implant.

For one aspect of the present invention, the desired dopant profile isachieved by bias ramping. In this embodiment, linear bias ramping tiedto the deposition rate is performed and in another embodiment,non-linear bias ramping also tied to the deposition rate is performed tomaximize the retained dose. The bias ramping rate may also be adjustedto be faster or slower than the deposition rate. Also, reverse biasramping may be performed for processes with net etching. The biasramping may also be combined with duty cycle ramping to prevent wafersurface arcing which may occur at high duty cycle bias ramping. The biasramping may also be combined with changing one or more processparameters to maximize the retained dose without a deep dopant profile.Examples of process parameters that may be changed include pressure, gasflow, gas composition, RF power, and temperature. It should be realizedthat other process parameters may be changed and the present inventionis not limited by the above referenced process parameters which areprovided as some examples. Also, profile engineering in any situation,deposition, etching or none, may be performed for high surfaceconcentration or a deep profile tail. An initial deposition may beperformed to reduce channeling. Open and closed loop control may beutilized for bias ramping. In-situ and ex-situ deposition measurementsmay be used in these control loops.

As shown in FIG. 4, the bias voltage increases during plasmaimplantation as indicated by the arrow length. The energy gain for eachion is equivalent to energy loss through the deposition layer so thation distribution within the substrate remains largely the same. Asdescribed above, the increase of bias voltage can be linear ornon-linear with time, depending on the deposition rate, which can alsobe linear or non-linear with time. Accordingly, s the retained dose ismaximized without a deep dopant profile.

FIGS. 5( a), 5(b), and 5(c) illustrate bias ramping combined with dutycycle ramping for achieving a higher duty cycle at lower ramping biasfor improving wafer throughput and photo-resist conditioning of thewafers. The bias voltage is applied only during the “on” period as shownin FIG. 5( a). However, the amplitude of the bias voltage may becontrolled separated by the controller 50. Also, the bias voltage maystart with a lower duty cycle as shown in FIG. 5( b) and end with ahigher duty cycle as shown in FIG. 5( c) or the bias voltage may startwith the higher duty cycle of FIG. 5( c) and end with the lower dutycycle of FIG. 5( b).

FIG. 6 illustrates the effects on the dopant profile when the biasramping is varied with respect to the deposition rate. In FIG. 6, line Ishows bias ramping that is slower than the deposition rate, line IIshows bias ramping that matches the deposition rate and line III showsbias ramping that is faster than the deposition rate. For loweringcontact resistance, a surface peaked profile is desired while a shallowor deeper profile is desirable for junction depth control andactivation.

FIG. 7 illustrates an embodiment of the present invention which utilizesreverse bias ramping in a process with net surface etching. The ionenergy is reduced to match the surface removal rate so that iondistribution within the substrate remains largely the same. Again, thedecrease of the bias voltage can be linear or non-linear with time,depending on the surface removal rate. Accordingly, the retained dose ismaximized without a deep dopant profile resulting.

FIG. 8 illustrates an embodiment of the present invention in which aninitial deposition may be made without a bias voltage being applied fora predetermined amount of time. When a single crystal substrate is used,the channeling tail may be reduced or even eliminated due to the angularspread of ions penetrating the initial deposition layer. Thereby,channeling may be reduced without the need of pre-amorphization implantwhich is critical for USJ formation in SDE doping. Also, the dopantlateral distribution can be modeled with the knowledge of incident ionenergy, flux and deposition rate.

In another embodiment, an amorphizing implant is performed. Anamorphizing implant will alter or destroy the long-range order of thepositions of the atoms in a crystal lattice. Dopant species such as B,P, or As or electrically-inert species such as H, N, He, Ar, Ne, Kr, orXe may be used to amorphize the crystal lattice. Larger atoms such as Sior Ge also may be used to amorphize the crystal lattice. The amorphousstate of the crystal lattice may later be changed from an amorphousstate to a crystalline state using an anneal. Thus, any damage caused bythe amorphizing implant may be repaired.

The amorphizing implant may be a pre-amorphizing implant (PAI) performedprior to doping the workpiece. An amorphizing implant also may occurduring the doping of the workpiece. This may be known as a“self-amorphizing” implant. For example, a “self-amorphizing” implantmay implant B and He at least partially simultaneously. The He willamorphizing the crystal lattice while the B dopes the workpiece.

Amorphizing the crystal lattice allows the resulting dopant profile tobe tailored. In one example, a He PAI is performed prior introducing thedopants. As the crystal lattice becomes amorphous and no longer has anordered structure, effects such as channeling of dopants, or theimplantation of ions substantially between the crystal lattice of thesubstrate, during later implantation may be prevented or reduced. Thisis because an organized or ordered crystal lattice may no longer exist.

The resulting amorphized region of the crystal lattice will form a sortof barrier hindering movement of subsequently implanted dopants. One ormore dopants will be implanted into the amorphized region and willsubstantially stop at the interface separating the amorphous andcrystalline regions (amorphous-crystalline interface). The dopant alsomay not diffuse past this amorphous-crystalline interface either duringimplantation or during an anneal.

Use of an amorphizing implant or PAI may allow formation of differentprofiles. If the amorphizing implant or PAI is performed prior toimplanting a dopant, the resulting dopant profile may either have asurface peak profile or a retrograde profile. In one embodiment, theamorphizing implant or PAI is performed to form an amorphous region nearthe surface of the workpiece. In the process, the amorphous-crystallineinterface also may be located near the surface of the workpiece.Subsequent implantation of dopants will result in a surface peak dopantprofile (similar to that illustrated in FIG. 6).

In yet another embodiment, a retrograde dopant profile is formed. Forexample, an amorphizing implant or PAI is performed so that theamorphous-crystalline interface is formed deeper in the workpiece.Subsequent doping implants will result in the formation of a retrogradedopant profile. FIG. 9 shows a retrograde junction profile. Asillustrated in FIG. 9, the peak dopant concentration does not occur atthe surface of the workpiece. Rather, the peak dopant concentrationoccurs below the surface of the workpiece.

Bias ramping may be combined with the amorphizing implant or PAI to forma retrograde profile. For example, a first bias level is applied to theworkpiece to amorphize a region of the crystal structure of theworkpiece. This amorphizing implant or PAI may be at a high energy and alow dose or low concentration. The dopant gas concentration in oneinstance is less than approximately 0.05%. Then, the bias voltage may beramped down to form the retrograde profile. Ramping down may beequivalent to reverse biasing. The bias voltage in one embodiment doesnot ramp down to zero. For example, the first bias level may beapproximately 350 V or 375 V and the second bias level may beapproximately 250 V. In another example, the first bias level may beapproximately 500 V and the second bias level may be approximately 350V. Other bias levels and dopant concentrations are possible and theembodiments of the process described herein are not solely limited tothe examples disclosed.

In one particular embodiment, a first bias level is applied to aworkpiece to amorphize a region of the crystal structure of theworkpiece. Then the workpiece has a second bias level applied to it,ramping down from the first bias level to the second bias level toimplant ions and form a retrograde dopant profile in the workpiece. Inone instance, only a single gas is used for both the amorphizing anddoping. This single gas is a dopant species. In another instance, atleast two gases are used. In this instance, a first gas is used at thefirst bias level. This first gas may be a dopant species or anelectrically-inert species. A second gas is introduced prior to or atleast partially simultaneously with the second bias level. This secondgas is a dopant species.

A retrograde dopant profile may be formed without an amorphizing implantor PAI. For example, the retrograde profile may be formed by controllingthe bias voltage. The bias voltage of may be ramped down to place thedopants at the proper depth in the workpiece and form the retrogradedopant profile.

Bias ramping and bias ramp rate adjusting may be performed to achieve aretrograde profile. This ramping may be linear or non-linear ramping.Increased bias voltage may result in a greater depth of amorphizationand, consequently, a deeper retrograde profile. Implant processparameters such as pressure, gas flow, gas composition, RF power, ortemperature also may be changed.

FIG. 10 shows a ramping bias voltage and resulting retrograde junctionprofile. There is a first bias level and a second bias level. In oneinstance, the first bias level may be applied for 25% of the totalimplant, the second bias level may be applied for 25% of the totalimplant, and the ramping period may be 50% of the total implant. Aretrograde dopant profile will be formed. FIG. 11 shows a non-rampingbias voltage and resulting surface peak dopant profile. Only a singlebias voltage is applied. This results in a surface peak dopant profile.The difference in depth between the dopant profiles in FIGS. 10 and 11may be the channeling component. This channeling tail that extends deepinto the dopant profile may be eliminated if an amorphizing implant orPAI is performed.

While this invention has been described in conjunction with the specificembodiments outlined above, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, the embodiments of the invention as set forth aboveare intended to be illustrative, not limiting. Various changes may bemade without departing from the spirit and scope of the invention asdefined in the following claims.

1. A method for plasma processing of a workpiece comprising: forming aplasma containing ions of a dopant gas; applying a first bias level tosaid workpiece; and ramping from said first bias level to a second biaslevel to implant said ions, said implant is configured to form aretrograde dopant profile in said workpiece.
 2. The method of claim 1,wherein said ramping comprises non-linear ramping.
 3. The method ofclaim 1, wherein said ramping comprises linear ramping.
 4. The method ofclaim 1, wherein a rate of ramping is adjusted.
 5. The method of claim1, further comprising amorphizing a region of a crystal structure ofsaid workpiece.
 6. A method for plasma processing of a workpiececomprising: forming a plasma containing ions of a first gas; applying afirst bias level to said workpiece; amorphizing a region of a crystalstructure of said workpiece at said first bias level; and ramping downfrom said first bias level to a second bias level to implant said ions,said implant configured to form a retrograde dopant profile in saidworkpiece.
 7. The method of claim 6, wherein said ramping down comprisesnon-linear ramping.
 8. The method of claim 6, wherein said ramping downcomprises linear ramping.
 9. The method of claim 6, wherein a rate oframping is adjusted.
 10. The method of claim 6, further comprisingpreventing channeling of said ions in said crystal structure of saidworkpiece.
 11. The method of claim 6, wherein said method furthercomprises changing at least one implant process parameter.
 12. Themethod of claim 11, wherein said at least one implant process parametercomprises pressure, gas flow, gas composition, RF power, andtemperature.
 13. The method of claim 6, wherein said ramping downcomprises reverse biasing of said workpiece.
 14. The method of claim 6,wherein said workpiece is a semiconductor wafer.
 15. The method of claim6, further comprising introducing a second gas prior to said rampingdown, said second gas containing a dopant species.
 16. The method ofclaim 6, wherein said first gas is of at least one of anelectrically-inert species and a dopant species.
 17. A method for plasmaprocessing of a workpiece comprising: forming a plasma containing ionsof a first gas applying a first bias level to said workpiece;amorphizing a region of a crystal structure of said workpiece at saidfirst bias level; and ramping said first bias level to a second biaslevel to implant said ions, said implant configured to form at least oneof a surface peak dopant profile and retrograde dopant profile in saidworkpiece.